The present invention relates to an exposure process for forming a resist pattern by exposing and developing a resist film deposited on a wafer in a manufacturing process of a semiconductor device, and more particularly to an exposure process monitoring method suited for controlling the exposure process.
FIG. 2 shows a flow of a conventional lithography process.
A resist pattern is formed by applying a resist which is a photosensitive material on a substrate such as a semiconductor wafer with a predetermined thickness, reducing and exposing a mask pattern by using an exposure tool (2050), and then developing the mask pattern (2051). The formed resist pattern is subjected to dimension checking by scanning electron microscopy (critical dimension-scanning electron microscopy or CD-SEM) equipped with a dimension measuring function (2052).
A conventional processing procedure by use of the critical dimension-scanning electron microscopy comprises, for example, acquiring an electron beam image of a region containing sites in which dimension accuracy is strictly controlled (2053), measuring the dimension (2054), deciding whether or not the dimension satisfies standards (2055), and then altering an exposure dose of an exposure tool if it is not satisfied (2056, a correction dose of the exposure dose is ΔE). For example, in the case of a positive type resist, if a resist dimension is too large, the exposure dose is increased, and if a resist width is too small, the exposure dose is decreased. The exposure dose to be increased or decreased is often determined on the basis of experience and hunch of an operator.
FIG. 3 shows a relation between a resist pattern and a film pattern after etching (p. 255 “ELECTRON BEAM TESTING HANDBOOK”, research material for industrial application of charged particle beam at 98 th meeting of 132 nd committee of Japan Society for the Promotion of Science). There is a certain relation between a shape of the resist pattern and a shape of the film pattern after the etching if etching conditions are similar. The resist pattern must have a predetermined shape to obtain a film pattern of a predetermined shape.
At the time of starting manufacturing of a semiconductor substrate of a new type or the like, before a product wafer is fed, a wafer is prepared in which a pattern is baked by changing a focal position and an exposure for each shot (exposure unit for one round) (such a wafer is normally called a focus & exposure matrix wafer (FEM wafer)), and dimensions of a resist pattern of each shot are measured. In addition, “condition finding work” is carried out to find a focal position and an exposure dose which enable acquisition of a predetermined resist pattern shape by cutting the wafer to investigate its shape of cross section or the like. By this work, a best exposure dose and a best focal position are decided, and the product wafer is subjected to exposure under such conditions.
With time, however, various process fluctuations (drifting of various sensors of the exposure tool, a change in photosensitivity of the resist, a variance in post exposure bake (PEB) temperature or the like) may occur to disable acquisition of a resist pattern of a proper shape under the conditions decided by the condition finding work. It is a role of the aforementioned dimension measurement (step 2052) that detects such disability. According to a conventional technology, compensation has been tried for process fluctuations by using the dimensions as a barometer thereof and correcting the exposure dose. Japanese Patent Application Laid-Open No. 11-288879 is available as a document regarding the conventional technology.
According to the conventional technology, to detect and counter process fluctuations, a method has been employed to investigate a dimension value of a line width or the like by using the CD-SEM, and to correct an exposure dose if the dimension value does not satisfy a standard.
However, recent micronization of semi-conductor patterns has been accompanied by very small fluctuation permissible amounts of the exposure dose and the focal position, creating a situation in which it is difficult to maintain the process within a proper range only by correcting the exposure dose. For example, it is now required to control an exposure dose fluctuation to 8 to 10% or lower, and a focal position fluctuation to 200 to 300 nm or lower at a node of 65 nm. To realize this control, information quantitatively indicating the process fluctuations, i.e., fluctuation amounts: deviation m joules of the exposure dose and deviation nm of the focal position, must be accurately quantified.
In the conventional technology, the fluctuation of the focal position may be overlooked (∵ fluctuation of focal position is not always accompanied by dimension fluctuation), and detection of exposure dose deviation is far from accurate (∵ dimension fluctuation may occur due to deviation of focal position). Further, even when the focal position should originally be corrected, the exposure dose is corrected, and thus there is apparently a case that a resist pattern of a proper shape cannot be obtained. Therefore, it is impossible to maintain the proper exposure process by the conventional technology.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.